FIG. 1 shows a pulse noise average value measuring and display device which is used in a conventional field intensity measuring instrument or field intensity meter. An input signal from an input terminal 11 is adjusted by a variable attenuator 12 to a proper level and is then supplied to a frequency converter 13. The frequency converter 13 is supplied with a local signal from a local oscillator 14, and the input signal converted by the frequency converter 13 to an intermediate frequency is amplified by an intermediate-frequency amplifier 15. By a suitable selection of the frequency of the local signal from the local oscillator 14, an input signal of a desired frequency can be obtained as the output of the intermediate-frequency amplifier 15. From the output of the intermediate-frequency amplifier 15 is extracted by a band-pass filter 16 a specified frequency component (9 kHz or 120 kHz) according to the rules of the Comite International Special des Perturbations Radioelectriques (CISPR), and the output of the band-pass filter 16 is provided to a linear detector (or envelope detector) 17, wherein its peak value is detected, and the detected output is integrated by an integrating circuit 18. The integrated output is amplified by an amplifier 19 and its level is displayed on a display 21.
When such impulsive noises 22 as shown in FIG. 2A are provided as input signals to the input terminal 11, the linear detector 17 yields an impulse 23 corresponding to the waveform of each impulsive noise at one polarity side as shown in FIG. 2B. Such impulses are averaged by the integrating circuit 18 as depicted in FIG. 2C. That is, the area of the impulse 23 and the area of that portion of the integrated output above the zero level become equal to each other in FIG. 2A-2D.
In the case where the pulse width is remarkably small and the pulse interval T.sub.1 is long relative to the peak value of the impulsive noise 22, the level of the integrated output becomes very low. For instance, when the pass frequency of the band-pass filter 16 is 120 kHz, the smallest pulse width W.sub.1 of the pulse 23 available from the linear detector 17 is 0.9 .mu.S (1/120 kHz). If the repetition frequency of the impulsive noise 22 is 100 kHz, the output of the integrating circuit 18, that is, the average value of the pulse 23, becomes 90 .mu.V, even if the peak value V.sub.p1 of the pulse 23 is 1 V. Thus the level difference between the peak value of the pulse 23 and the integrated output is as large as 81 dB.
As mentioned above, when the impulsive noise 22 is very short and its period of generation T.sub.1 is long, the output level of the integrating circuit 18 drops very low, sometimes, close to or below the noise level. If the attenuation of the variable attenuator 12 is set small so as to avoid such a situation, the frequency converter 13 and the intermediate-frequency amplifier 15 are supplied with pulse signals of large peak values and become saturated, providing waveform distortions. For these reasons, it has been impossible to accurately measure the average value of pulse signals of extremely long pulse intervals.
The integrating circuit 18 performs integration through use of a CR circuit. To remove ripples from the integrated output and hence sufficiently smooth it, the CR time constant for the integration needs to be selected sufficiently larger, for example, about 100 times larger than the interval T.sub.1 of the impulsive noise 22. Since the interval T.sub.1 is 1 sec or so in some cases, the above-mentioned time constant is usually set to approximately 100 sec. This poses another problem that a large amount of time is needed to obtain accurate measured values.
The prior art uses a spectrum analyzer to measure the frequency components of impulsive noises. The conventional spectrum analyzer is shown in FIG. 3. The input signal from the input terminal 11 is adjusted by the variable attenuator 12 to a proper level and is then applied to the frequency converter 13. The frequency converter 13 is supplied with a local signal from the local oscillator 14 as well. The oscillation frequency of the local oscillator 14 is swept by a ramp signal from a ramp signal generator 24. Consequently, the frequency of the received input is swept. The intermediate-frequency amplifier 15 is capable of varying its pass frequency and gain and is provided with a pass frequency varying part 15a and a gain varying part 15b. The output of the amplifier 15 is detected by the linear detector 17, the detected output of which is periodically sampled by an A-D converter 24 and each sample value is converted to a digital signal. The digital signal is written into an image memory 26. When data of one frame of a display 27 is stored in the image memory 26, the display 27 reads out the stored contents of the image memory 26 and displays them as an image. The attenuation of the variable attenuator 12, the passing frequency and gain of the intermediate-frequency amplifier 15, the operation of the ramp signal generator 24, the operation of the A-D converter 25 and the operation of the display 27 are placed under the control of a control circuit 28 equipped with a CPU. Various parameters for measurements can be entered and set in the control circuit 28 through a keyboard 29.
For example, as shown in FIG. 4A, the frequencies and levels of received input signals are displayed as waveforms on a display screen 27a of the display 27 with the abscissa representing frequency and the ordinate level. In the case of observing temporal variations of one of the input signals, for example, a signal which seems to be an impulsive noise, the sweeping of the local oscillator 14 is stopped and its oscillation frequency is set to a value corresponding to the frequency component desired to observe, that is, the signal is received in what is called a zero span mode. In this instance, for example, as shown in FIG. 4B, the input signal waveform is displayed on the screen 27 of the display 27 with the abscissa representing time. In the prior art, however, the spectrum analyzer is not equipped with a function for obtaining the average value of impulsive noises.
An object of the present invention is to provide an average value measuring device capable of measuring the average value of pulse signals with accuracy.
Another object of the present invention is to provide an average value measuring device capable of measuring the average value of pulse signals or the like accurately and rapidly.